Immunocastration is an alternative method used to replace surgical castration commonly performed in swine farms. In boars, the main effects of immunocastration are reduction of gonadotropin-releasing hormone (GnRH) and the resulting inhibition of testicular function. The aim of this study was to evaluate immunocastration efficacy in pre-pubertal boars vaccinated with a recombinant GnRH protein conjugated with Salmonella Typhimurium flagellin fljB (STF2). A total of 35 boars were assigned to three groups: the untreated group (n ＝ 5), the surgically castrated group (n ＝ 5), and the immunocastrated group (n ＝ 25). Pigs in the immunocastration group were immunized with the GnRH-STF2 vaccine at pre-pubertal ages 4 and 8 weeks. All experimental pigs were kept for 26 weeks before slaughter. Anti-GnRH antibody levels of immunocastrated pigs were significantly higher than those of untreated pigs (P ＜ 0.001). In contrast, testosterone levels of immunocastrated pigs were significantly lower than those of untreated pigs (P ＜ 0.001). Statistical significances were not found in the body weights and backfat thicknesses of untreated vs. immunocastrated pigs. Weights of the testes and epididymides of immunocastrated pigs were significantly lower than those of untreated pigs (P ＜ 0.001). Testicular tissues of immunocastrated pigs were severely suppressed compared with those of untreated pigs. In conclusion, immunization with the STF2-GnRH vaccine effectively induced immunocastration in pre-pubertal boars.